Switch MinGW build to use UNICODE too

Partial backport of musescore#7744

Author: shoogle

CMakeLists.txt

@@ -312,7 +312,7 @@ else (APPLE)
       set(CMAKE_C_FLAGS_RELWITHDEBINFO   "/MT /W4 /Zi /O2 /Ob1 /DNDEBUG")
       set(CMAKE_EXE_LINKER_FLAGS         "/DYNAMICBASE:NO")
    else (MSVC)
-      set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -std=gnu++11")
+      set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -std=gnu++11 -D_UNICODE -DUNICODE")
       if (MINGW)
          # -mno-ms-bitfields see #22048
          set(CMAKE_CXX_FLAGS         "${CMAKE_CXX_FLAGS} -mno-ms-bitfields")

libmscore/scorefile.cpp

@@ -447,16 +447,7 @@ bool MasterScore::saveFile(bool generateBackup)
                   dir.mkdir(backupSubdirString);
 #ifdef Q_OS_WIN
                   const QString backupDirNativePath = QDir::toNativeSeparators(backupDirString);
-#if (defined (_MSCVER) || defined (_MSC_VER))
-   #if (defined (UNICODE))
-                  SetFileAttributes((LPCTSTR)backupDirNativePath.unicode(), FILE_ATTRIBUTE_HIDDEN);
-   #else
-                  // Use byte-based Windows function
-                  SetFileAttributes((LPCTSTR)backupDirNativePath.toLocal8Bit(), FILE_ATTRIBUTE_HIDDEN);
-   #endif
-#else
-                  SetFileAttributes((LPCTSTR)backupDirNativePath.toLocal8Bit(), FILE_ATTRIBUTE_HIDDEN);
-#endif
+                  SetFileAttributesW(reinterpret_cast<LPCWSTR>(backupDirNativePath.utf16()), FILE_ATTRIBUTE_HIDDEN);
 #endif
                   }
             const QString backupName = QString(".") + info.fileName() + QString(",");